1. Field of Invention
The invention relates to a semiconductor memory device, and more particularly to a semiconductor memory element with a multi-level memory state.
2. Related Art
Memory is widely used in general electric devices. Most are DRAM, SRAM, or Flash memory. Application and architecture of electric devices determine the usage of the memory and the required capacity. Development of memory technology, such as FeRAM, MRAM and phase change memory technology, is ongoing.
A phase change semiconductor memory element stores data through resistance variation caused by phase change of materials. Regarding the phase materials, in the 1960s, S. R. Ovshinsky of the U.S. company ECD discovered that crystallization and amorphization of chalcogenide has a distinct difference in optics and electrical conductivity. It is capable of fast reversible transformation and has switching/memory application.
A phase change memory element is called a semiconductor memory because chalcogenide belongs to the VIA group in the Periodic Table Of Elements, and is a semiconductive material between metals and nonmetals. Adding some elements is required for specific purposes in practical use, for example, increasing amorphization/crystallization speed, or crystallization characteristics.
Phase change memories meet the need for large and fast storage operations and long storage time. It has the advantages of small volume, more storage data, and fast operation speed, and may store data more than ten years under 130° C. Therefore, a phase change memory element is a non-volatile memory element with great potential, having high read/write speed, high integrity, long endurance, low power consumption, and radiation hardness. Main technology trends focuses on higher record density and low power consumption through reducing memory cells.
However, besides increasing the memory density by reducing the area, multi-level/multi-state memory is another consideration. Thus, a single memory cell may have more than two memory states in the condition of not changing the component size.
In the related art, Tyler Lowrey (Ovonyx Inc.) provides a multi-state structure in a published company technology document. A memory cell with a single phase change layer is employed to obtain multi levels with different resistant values by controlling the reset current. However, the solution may have the problem of small current separation such that writing error occurs due to the current offset.
Also, U.S. Pat. No. 6,635,914 discloses a four level memory cell that belongs to the category of Programmable Metallization Cell Memory (PMCm). The cell is composed of a solid electrolyte layer and two electrodes. The conductivity of the solid electrolyte layer is changed by delivering an electrical field by the electrodes.
Phase change memory, MRAM, and FRAM are the main memory technology trends, which have the advantages of being non-volatile, high speed (close to the operation speed of DRAM and SRAM), large capacity, high integrity, high environment endurance, long storage time, etc. Furthermore, operation voltage is decreasing gradually. These memories may substitute Flash memory in the near future. Therefore, there is an urgent need for a new phase change memory element structure.